TABLE OF CONTENTS All measurements are subject to measurement error. By error is meant the difference between the observed value and the true value of the quantity being measured. Since true values are invariably unknown, the exact magnitude of the error involved in an analytical result is also invariably unknown. It is possible, however, to estimate the likely magnitude of such errors by careful study of the properties of the analytical system. The term 'analytical system' refers to everything that impinges on a measurement: the method, the equipment, the reagents, the analyst, the laboratory environments, etc. It is fair comment that a measurement is of no value unless there is attached to it, either explicitly or implicitly, some estimate of the probable error involved. The Analytical Methods Committee of the Royal Society of Chemistry1 has taken a very clear position on this question in saying that "analytical results must be accompanied by an explicit quantitative statement of uncertainty, if any definite meaning is to be attached to them or an informed interpretation made. If this requirement cannot be fulfilled, there are strong grounds for questioning whether analysis should be undertaken at all." It gives a simple example which illustrates the necessity for measures of uncertainty: "Suppose there is a requirement that a material must not contain more that 10 ?g g -1 of a particular constituent. A manufacturer analyses a batch and obtains a result of 9 ?g g -1. If the uncertainty... in the results...
